The present invention relates to a method of continuous dehydration of sludge wherein, prior to dehydration, water is mechanically removed from the sludge and the thus obtained sludge of cutting consistency is mixed with heated spheres. The invention also relates to an apparatus for carrying out such a method.
Before discussing the prior art it is observed that the processing of sludge is one of the major problems of the immediate future. The reason is that it is difficult to remove water from sludge (mechanically). Sludge consists of (very) small particles varying from roughly 1 to 50 .mu.m in size. When water is removed, these small particles are the cause of a clay-like structure with many pores. The water in said pores is difficult to remove by means of mechanical treatment (for instance by pressing). The content of organic matter also considerably influences the water removal properties. The source of the sludge determines its composition and consequently also its water removal properties. For instance, one may discriminate between sludge coming from the processing of polluted soil, the processing of construction and demolition rubble, (sewage) water purification installations, channel beds or industrial waste-water treatment installations.
Formerly, various kinds of sludge were used as agricultural fertilizers. Due to stricter environmental legislation and the manure surplus, this practice has virtually fallen into complete disuse. Nowadays the ensuing sludge streams are dumped or, if the sludge is heavily polluted, incinerated. At the moment the usual costs for dumping amount to 40 to 60 U.S. dollars per metric ton and incineration costs are about 250 U.S. dollars per metric ton. To keep the costs for the sludge disposal within limits, extensive water removal is advisable. Sludge from construction and demolition waste contains little organic matter, and it is generally easy to remove water mechanically to an approximate dry matter content of 65%. The sludge from soil decontaminators generally contains more organic matter, up to approx. 20%, and mechanical water removal is possible up to a dry matter content of 40-50%. The sludge from the purification of waste water sometimes contains more than 50% organic matter, so that mechanical water removal results at best in an approximate dry matter content of 20%.
Thus, for a considerable part, the processing costs for sludge are determined by the water content. A lower moisture content is practically only possible by evaporating water. However, evaporating water requires much energy, 2260 kJ per kg water. By way of comparison, this is equivalent to the amount of energy required to raise the temperature of 10 kg water by 53.degree. C.
In the majority of dehydration installations large quantities of hot air are used to withdraw water from the material. The costs for dehydrating sludge amount to approx. 100 U.S. dollars per metric ton. In a number of cases the sludge does not need to be dumped subsequent to dehydration, because the dehydrated sludge can be used for other purposes.
The method and apparatus described in the preamble are known from WO 91/001945. The known method is carried out at atmospheric pressure. In order to obtain a large heat transferring surface, metal spheres are added to the sludge. Prior to the addition of the spheres to the sludge, they are heated to approx. 150.degree. C. by means of hot gas. Because the dehydration process takes place in a revolving drum with a screw conveyor, mixing of the spheres with the sludge is assured, with good heat transfer. Finally, the spheres are separated from the sludge. According to this known method the apparatus is able to process a sludge stream of 5.2 metric tons per hour having a dry matter content of 15%, yielding 95% dry matter in four minutes. An important disadvantage of the known method is the large amount of hot air/gas required, which obviously means considerable energy consumption.